The present invention relates to lasers and particularly to a laser diode pumped solid state laser for producing laser emission at one or more laser wavelengths.
Lasers are devices that generate or amplify light. The beams of radiation that lasers emit or amplify have remarkable properties of directionality, spectral purity and intensity. These properties have already led to an enormous variety of applications. The essential elements of the laser device are (1) a laser medium consisting of an appropriate collection of atoms, molecules, ions or, in some instances, a semiconducting crystal; (2) a pumping process to excite these atoms, molecules, etc., into higher quantum mechanical energy levels; and (3) suitable optical feedback elements that allow a beam of radiation to either pass once through the laser medium that is in a laser amplifier, or bounce back and forth repeatedly through the laser medium, as in a laser oscillator. The laser contains an optical cavity or resonator which is defined by highly reflecting surfaces which form a closed round-trip path for light. The laser medium is contained within th optical cavity.
The elements of a laser come in a great variety of forms and fashions. One type of laser medium that has numerous advantages compared to others is the solid state laser medium consisting of a laser crystal with one or more dopant ions that is, generally, optically pumped. The optical pump may be an incoherent source, such as cw lamps, including tungsten filament lamps or arc lamps, or pulsed lamps, such as flashlamps, or a monochromatic laser source. Monochromatic laser pump sources include ion lasers or dye lasers or semiconductor laser diodes, such as, an aluminum gallium arsenide (AlGaAs) laser diode operating at approximately 808.5 nm pumping a Nd:YAG solid state laser material. As an example of this type of laser, see the article "Efficient Laser Diode Pumped Nd Lasers" by Richard Scheps in Applied Optics, vol. 28, pp. 89-91 (January 1989).
In order for the optical pumping process to be effective, the photons incident on the laser medium must have certain properties. In particular, the pump radiation must be of a wavelength which is absorbed by the laser medium to generate either directly or indirectly the required population inversion for the desired laser transition.
Laser diode pumping of Nd:YAG lasers is well recognized. The laser diode output radiation must substantially match the desired absorption wavelength of the Nd:YAG laser medium, which in general, corresponds to a wavelength of 808.5 nm. This matching of the laser diode emission wavelength with the absorption wavelength and bandwidth of the Nd:YAG laser material at 808.5 nm is required for relatively efficient operation, both in terms of pumping efficiency, by which is meant the efficiency by which pump photons populate the upper laser level, and in terms of overall electrical power consumption by the pump diodes themselves to generate a specific amount of optical laser power from the Nd:YAG laser material. The absorption bandwidth of Nd:YAG is approximately 1 nm. It is both difficult and expensive to fabricate laser diodes or laser diode arrays with the proper power, bandwidth and center wavelengths required for efficient pumping of Nd:YAG laser media.
The typical AlGaAs semiconductor material pumping diodes have a naturally broad wavelength range over which they can produce radiation. That is to say, although the bandwidth of an individual AlGaAs semiconductor laser is relatively narrow, the center wavelength can vary over 50 nm or more from one device to another. This is due, partially, to the fact that the center wavelength is highly sensitive to the aluminum concentration in the AlGaAs active layer. An additional problem for AlGaAs laser diode pumping of Nd:YAG lasers is that the junction temperature of the pump diode requires accurate thermal control for efficient operation of the Nd:YAG laser due to the narrow absorption bandwidth at 808.5 nm since there is a well known shift of the semiconductor laser central wavelength of approximately 0.3 nm per degree Centigrade change in junction temperature.
Because the nature of the semiconductor laser diode is such that approximately 60 percent or more of the electrical power used to drive the diode is dissipated in the diode junction as heat, junction temperatures will rise dramatically as the device is turned on. Thermal control, therefore, is extremely important in terms of maintaining the appropriate temperature of the semiconductor diode junction to produce the proper center wavelength for the laser diode. Practical operating temperature ranges run generally from 10.degree. C. to 30.degree. C., and since center wavelength shifts with junction temperature of 0.3 nm per degree C., the 20 Centigrade degree operating temperature range for the semiconductor diode laser can produce at most a 6 nm shift in wavelength. Therefore, if the semiconductor laser diode that is intended to pump the Nd:YAG laser gain medium is not within just a few nanometers of 808.5 nm, then it is impractical to resort to cooling or heating of the semiconductor laser diode to try to achieve the proper emission wavelength from the semiconductor laser. In addition, control of the diode junction temperature over a range of semiconductor laser diode pump powers is cumbersome and adds complexity to the overall system.
The narrow absorption linewidth of Nd:YAG also increases the difficulty of modeling both the inversion profile resulting from volumetric deposition of the polychromatic pump excitation as well as modeling the effects of anticipated spectral shifts in the pump source due to aging; see for example of R. Scheps and J. F. Myers, "Performance and Aging of High Powered 2-D Laser Diode Arrays," Applied Optics, vol. 29, p. 341 (1990).
While individual laser diodes may have their central emitting bandwidth adjusted by temperature control of the junction, this capability does not apply to laser diode arrays that are used as the pumping source. These arrays generally consist of multiple semiconductor laser diode stripes on a single monolithic semiconductor laser substrate. The bandwidth and central wavelength variation from stripe to stripe must be considered and generally leads to a much broader emission bandwidth and a much less well-defined central bandwidth. From a pragmatic viewpoint, a specification requiring a high power, narrow band diode pumping array dramatically increases the cost of purchasing such a pump. Because, in practice, the diode pump array is often the single most expensive component in the laser head, the total cost can easily become prohibitive.
To address the issue of cost effective pumping, hosts for the Nd ion have been sought in which the width of the 800 nm absorption spectrum is increased. Two examples of such host materials are Nd:BEL, see the article by R. Scheps et al., "End-Pumped Nd:BEL Laser Performance," in Optical Engineering, vol. 27, p. 830 (1988), and Nd:YVO.sub.4 ; see the article of R. A. Fields et al., "750 mW, 1.06 .mu.m cw TEM.sub.00 Output from a Nd:YVO.sub.4 Laser End Pumped by a Single 20-Stripe Diode Array," in Advances in Laser Science-III, American Institute of Physics Conference Proceedings, Atlantic City, N.J., Vol. 172, pp. 20-22 (1987). Both of these Nd doped crystals have been diode pumped and have a significantly broader absorption bandwidth than Nd:YAG.
While Nd:BEL and Nd:YVO.sub.4 may be promising, the largest practical absorption bandwidths for singly doped neodymium crystals are on the order of 5-10 nm at the 800 nm emission bandwidth for AlGaAs semiconductor laser diodes. The search for hosts where a neodymium absorption band is broader generally has led to materials where the neodymium emission cross-section is drastically compromised to the point where the emission becomes highly inefficient. In other words, although the pumping efficiency has increased, (a greater fraction of the pump photons are absorbed by the gain medium and subsequently populated the upper laser level), the extraction efficiency, (the ability to utilize the inverted population to produce laser emission), has been lowered dramatically.
An alternative approach to achieve high pump efficiency while maintaining high extraction efficiency is to co-dope the neodymium host material with an activator ion such as Cr.sup.3+. By co-doping with chromium, the very broad absorption spectrum in the red (near 660 nm) can be advantageously utilized to increase the pump efficiency. Co-doping also improves the efficiency for flashlamp pumping. An example of a flashlamp pumped design is given in U.S. Pat. No. 4,837,769 by S. Chandra et al. called "Phase Conjugated Slab Laser Designator." See also the article by A. A. Kaminskii et al., "Luminescence and Stimulated Emission of Nd.sup.3+ Ions in Gd.sub.3 Sc.sub.2 Ga.sub.3 O.sub.12 Crystals," Physics Status Solidi (a), vol. 34, p. K109 (1976). Relative to Nd.sup.3+ the Cr.sup.3+ ion absorbs most of the pump photons and is excited to the .sup.4 T.sub.2 state. The excited Cr.sup.3+ ion subsequently undergoes a rapid and efficient excitation transfer between the .sup.4 T.sub.2 state of Cr.sup. 3+ and the .sup.4 F.sub.3/2 Nd level. The .sup.4 F.sub.3/2 Nd ion excited state is the upper laser level. This excitation transfer process therefore directly populates the upper laser level of the neodymium ion, from which lasing at 1.06 .mu. is observed.
The efficiency of transfer of energy from the Cr.sup.3+ ion to the Nd.sup.3+ ion has been measured to be 87 percent, by which it is meant that 87 percent of the pump energy which is initially absorbed by the Cr.sup.3+ ion is transferred to the upper laser level of the Nd.sup.3+ ion. Some of the excitation that initially resides in the Cr.sup.3+ ion is lost through fluorescence, but it is to be noted that the Nd.sup.3+ ion has strong absorption bands in the same spectral range as the Cr.sup.3+ fluorescence. Therefore, an additional excitation transfer process occurs through absorption of the Cr.sup.3+ fluorescence by the neodymium ion.
Previous results on laser operation of a Cr,Nd:GSGG laser have been reported. The host GSGG is gadolinium scandium gallium garnet and has the chemical formula Gd.sub.3 Sc.sub.2 Ga.sub.3 O.sub.12. A report by D. Pruss et al., "Efficient Cr.sup.3+ Sensitized Nd.sup.3+ GdScGa-garnet Laser at 1.06 .mu.m" which appeared in Applied Physics B, vol. 28, p. 355 (1988) identified the mechanism for excitation transfer. Only a krypton ion laser was used as the pumping source which emitted radiation at 647.1 nm. The use of a krypton ion laser make applications of a laser pumped Cr,Nd:GSGG laser based on the work of Pruss et al. highly impractical since for one thing, the ion laser is less than 0.01 percent efficient in terms of converting electrical efficiency to optical output. The krypton ion laser requires enormous cooling power and very high amperage laser drive current, typically over 100 amps at 220 volts to operate the ion laser. In addition, the pumping laser is very large and easily damaged because it contains a glass plasma tube so that portability, compactness and other features which might otherwise make this device more useful cannot be achieved with a krypton ion pump.
U.S. Pat. No. 4,740,975 by Abrahams et al. and U.S. Pat. No. 4,802,180 by Brandle, Jr. et al. also have discussed a laser pumped Cr,Nd:GSGG laser. Both of these patents are similar and focus on addressing the concepts of growing the laser medium crystal itself and specific techniques required to grow defect-free co-doped Cr,Nd:GSGG laser crystals. Both patents are slab-shaped designs to be side pumped and completely overlook the concept of an end-pumped Cr,Nd:GSGG laser.
There have been numerous patents issued for the concept of laser diode pumped neodymium doped lasers. Two patents have specifically mentioned diode pumped Cr,Nd:GSGG lasers. These are U.S. Pat. No. 4,901,330 by Wolfram et al. and U.S. Pat. No. 5,029,335 by Fisher et al. The Wolfram patent is concerned with generating optically pumped radiation from a laser diode array and mentions co-doped Cr,Nd:GSGG as a possible laser medium which can be utilized. However, the particular invention of Wolfram et al. would preclude good end pumping. More importantly, Wolfram et al. overlook the fact that the AlGaAs diodes which were used in the neodymium pumped laser materials, which will emit in the 800 nm range, are entirely useless for the co-doped material in the sense that they only pump the neodymium ion. These AlGaAs diodes, because of their emission bandwidth, can only pump the neodymium in the co-doped Cr,Nd:GSGG and cannot excite the Cr.sup.3+ ion. Furthermore, Wolfram et al. fail to acknowledge that different pump wavelengths are required for the co-doped material in order to take advantage of the broad absorption band in the visible. Nor do they note the entirely different wavelength dependence and lack of wavelength matching criteria inherent in the Cr.sup.3+ co-doped material relative to the singly doped neodymium materials that they address in their invention.
The Fisher et al. patent describes an improved heat dissipating package for a laser diode pumped neodymium doped laser. It mentions the problems involved in a wavelength mismatch between the pump emission and the laser medium absorption and specifically addresses the thermal shift in the emission wavelength of the pump laser diode that must be controlled adequately to maintain high pump efficiency. However, it shows a resonator which does not allow compact, efficient, end pumped operation when pumped by laser diodes. The Fisher et al. patent mentions co-doped Cr,Nd:GSGG as a potential laser medium which would be suitable in their inventive concept. The lack of sensitivity of the pump efficiency to the pump wavelength is not mentioned in the Fisher patent which is a benefit of using the co-doped material. The lack of wavelength sensitivity afforded by co-doped materials may obviate the need or motivation for Fisher et al.s' temperature compensation scheme. That is, although heat removal is nearly always a design consideration, the invention described by Fisher et al. is for the rapid and efficient removal of heat from the laser diode in order to match the laser diode wavelength to the narrow absorption linewidth in neodymium. Considering the broad absorption linewidth of chromium in the visible region of the spectrum, there is little, if any, need to control the wavelength of such a co-doped Cr,Nd:GSGG diode by thermal means.
An example of the types of problems which arise when dealing with the very narrow absorption bandwidth in neodymium doped materials, coupled with the broader emission profile and the shift in central wavelength for laser diodes, is represented in U.S. Pat. No. 4,942,582 by Kintz et al. This patent demonstrates a high output power laser which operates in a single longitudinal mode. Such operation is achieved because the end pumped laser diode fluence is absorbed in a very short axial distance within the crystal near the face that is pumped. In order to accomplish this, a laser diode source with a very narrow bandwidth is needed as the absorption depth in the crystal must be narrow. This requirement, of course, makes the selection of the diode and the operating conditions under which effective pumping of this material can occur (such as junction temperature) much more constrained and more complex, and as a consequence less efficient.
Thus in accordance with this inventive concept a need has been recognized for a neodymium laser that can be diode pumped with a high pump efficiency but is insensitive to the exact pump wavelength and bandwidth of the pump diodes.